nixos: iso-image: use syslinux bootloader for USB booting support

This changes the bootloader for iso generation from Grub to syslinux. In addition this adds USB booting support, so that "dd" can be used to burn the generated ISO to USB thumbdrives
instead of needing applications like UnetBootin.

Most of this is based on the ISO generation methods that is used for ArchLinux.
It's already enabled for the installer. It's that you added the usb, and now needs to be enabled for the installer.